1. Field of the Invention
The present invention relates to an apparatus and method for Time-Varying Cyclic Delay Diversity (TV-CDD) in a wireless communication system. More particularly, the present invention relates to an apparatus and method for varying a scheme of applying a cyclic shift of a preamble zone and data zone in an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system employing a TV-CDD scheme.
2. Description of the Related Art
The term diversity refers to the use of a plurality of versions of a given signal. By employing diversity, a diversity effect may be realized whereby, even when some versions of the signal experience a problem in transmission, reception can still be facilitated with the remaining versions of the signal. A wireless channel that a signal goes through during transmission in a wireless communication system has a different characteristic depending on a space, a time, a frequency, a phase, etc. A receiver can obtain the diversity effect using a plurality of versions of the same signal that differ in at least one characteristic as the same signal goes through a channel.
More specifically, in a wireless channel environment, there are several propagation paths for a signal between a transmitter and a receiver. Each propagation path can apply time-varying fading to a corresponding signal and can have a different construction and at least one characteristic depending on a position of a receive antenna. Therefore, the receiver can obtain a diversity effect based on at least one of space, time, and frequency using the construction and at least one characteristic of the propagation path.
As such, the diversity effect can increase by transmitting several versions of a signal. That is, a transmitter generates and transmits a plurality of versions of a signal each having at least one different characteristic of space, time, frequency, etc. Consistent with the transmission diversity method described above, there is a method for separating the signal into similar versions of the signal that are different from each other in at least one of space, time, frequency, etc. for transmission and a method for controlling a transmitted signal to intentionally have a specific property during reception.
Cyclic Delay Diversity (CDD) is a type of a transmission diversity scheme. The CDD is a method for increasing a frequency selectivity of an effective channel seen from a received signal. More specifically, the CDD applies, by a different amount of time, a cyclic shift to each antenna element transmitting the same symbol if one data symbol is transmitted from a transmit antenna to two or more antenna elements, that is, if a Multiple Input Multiple Output/Multiple Input Single Output (MIMO/MISO) channel is converted to a Single Input Multiple Output/Single Input Single Output (SIMO/SISO) channel.
FIG. 1 illustrates an example of cyclically shifting, by a D-sample duration, samples constituting an Orthogonal Frequency Division Multiplexing (OFDM) symbol in an OFDM system according to the conventional art.
Referring to FIG. 1, an original version of an OFDM symbol comprised of a data duration with N samples and a Cyclic Prefix (CP) duration with L samples, and a cyclically shifted version of the OFDM symbol, are illustrated. Here, a subscript of a sample ‘S’ denotes a time index. In addition, CDD can identify that, if samples constituting a symbol are out of an original symbol duration due to a set shift, the samples are shifted before the symbol.
The CDD scheme provides the principal effects of a conversion of space diversity into frequency diversity and a constructive aggregation of a transmitted signal. First, the CDD scheme has an advantage of having no influence on a shift spread of an effective channel because a cyclic shift of a given OFDM symbol does not cause Inter-Symbol Interference (ISI) on a next symbol and simultaneously increases a frequency selectivity of a channel, thereby increasing a frequency diversity effect. In particular, the diversity effect is increased in a channel having a relatively small frequency selectivity. Second, the CDD scheme can decrease a probability of, during transmission, destructively combining a signal transmitted by each antenna and can increase a probability of constructively combining the signal, by controlling a shift length for each antenna element when using CDD. Because of the reasons stated above, the CDD is in use or is being contemplated for use in a variety of OFDM systems such as systems based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and 802.16 standards.
However, the unlimited use of CDD in an OFDM system can negatively influence functions of channel estimation of a receiver, timing synchronization, etc. as described below.
First, because a time average is taken for a performance improvement in channel estimation of a receiver, if there is a change of a parameter such as a transmit power or number of transmit antenna elements depending on time (i.e., a symbol) within one frame when using CDD, the performance of a channel estimation scheme of a receiver can deteriorate.
Second, if a length of a cyclic shift of the CDD increases more than a preset level, that is, if a frequency selectivity of an effective channel increases more than a preset level, a channel coherence bandwidth decreases compared to a pilot interval of a given OFDM system, thus leading to a failure of a follow-up channel estimation of a receiver.
Third, the use of CDD can lead to a variation of a multipath profile of an effective channel seen from a receiver, thus causing ISI due to an error of a selection of a Fast Fourier Transform (FFT) sampling point of a receiver.
Estimating an FFT sampling point, that is, a start point of a time-axis sample duration (i.e., an FFT window) selected for input to an FFT unit in a receiver of an OFDM system, uses a correlation characteristic of a known part (e.g., a preamble and midamble) of a received signal.
FIG. 2 illustrates an example of a variation of a multipath profile of a received signal caused by CDD in a receiver of an OFDM system according to the conventional art. In FIG. 2, a channel multipath profile is viewed based on a start point of a data duration (i.e., an FFT sampling point).
Referring to FIG. 2, a channel multipath profile 205 seen from a correlator of a receiver is equal to a sum of multipath profiles 201 and 203 for an original signal and each signal applying a cyclic shift. The aforementioned suggests a possibility of a problem that, if a length of a cyclic shift of CDD is more than a preset level, a channel shift spread seen from the receiver can exceed a CP duration and a possibility of a problem that the use of CDD can lead to an increase of a probability of an error of a selection of an FFT sampling point. The conditions of generating the error of the selection of the FFT sampling point can be described through an example of a system in which two antenna elements each use a different cyclic shift in a single path channel, as described below with reference to FIG. 3.
FIG. 3 illustrates a process of generating an error of a selection of an FFT sampling point of a receiver and its influence in a 2-antenna OFDM system employing a CDD scheme according to the conventional art.
Here, a 0th CDD signal 301 is equal to a reference signal that does not have a cyclic shift, and a 1st CDD signal 303 is equal to a signal that does have a cyclic shift. A path 0 (305) represents a channel response corresponding to the 0th CDD signal 301, and a path 1 (307) represents a channel response corresponding to the 1st CDD signal 303. On the assumption that the two antenna elements have the same gain, a two equal-gain path channel 309 comprised of the path 0 (305) and path 1 (307) having a time difference of as much as a cyclic shift is seen from a correlator of a receiver. The two paths each suffer different fading. Thus, if the 0th CDD signal 301 suffers a deep fade and the 1st CDD signal 303 does not, only the path 1 (307) can be significantly seen in an estimated channel response.
Thus, a receiver can select a position of the path 1 (307) as an FFT sampling point. That is, the receiver can apply an FFT window 0 (311) to a time-axis sample duration. However, in the system, an accurate time-axis sample duration corresponds to an FFT window 0 (311) having a position of the path 0 (305) as an FFT sampling point and thus, if an FFT window 1 (313) is applied to the time-axis sample duration, ISI is generated including a CP duration of a next symbol by as much as a cyclic shift length. An amount of ISI generated as described above increases in proportion to the cyclic shift length. In a multipath channel, a possibility of not selecting a dominant path of the 0th CDD signal 301 but instead selecting a dominant path of the 1st CDD signal 303 as an FFT sampling point increases. This is because a position of a secondary path of the 0th CDD signal 301 is similar to a position of a dominant path of the 1st CDD signal 303, thus leading to an increase of a correlation value of a corresponding position.
Such a problem of an error of a selection of an FFT sampling point does not take place in a general multipath environment but takes place in the case of applying the CDD scheme. In the case of not applying the CDD scheme, although a reference path in a general multipath channel suffers deep fading and thus a different path on time is selected as an FFT sampling point, a new reference path becomes an accurate FFT sampling point because a signal energy corresponding to a previous reference path is negligibly weak. In the case of applying the CDD scheme, a reference FFT sampling point is the same for all CDD signals and therefore, an error of a selection of a time-axis sample duration takes place if a path not applying a cyclic shift is lost and a position of a different path is taken as an FFT sampling point.
Among the aforementioned problems, the problem that occurs because the time average is taken for a performance improvement in the channel estimation of the receiver can be solved by limiting a time-dependent variation of a signal power, number of antennas, etc. in a transmitter. In addition, the problem that occurs because the length of the cyclic shift of CDD increases more than a preset level can be reduced if a maximum allowance value of a length of a cyclic shift is set considering a minimum value of an expected channel coherence bandwidth and a pilot interval on the standard of a given system. However, there is a persistent problem of a possible error of a selection of an FFT sampling point at the time of applying the CDD scheme. More particularly, when a length of a cyclic shift is high enough or a shift spread of a channel is high, there is a problem that a deterioration of performance caused by ISI is much higher than a diversity effect obtained by applying the CDD scheme.